1. Field of the Invention
This invention relates to a type of gun known as a two-stage light gas gun, which is designed to fire projectiles at very high speeds.
2. Background of the Invention
A light gas gun is designed to shoot projectiles at very high speeds by utilizing a high-pressure gas of low atomic number, typically either hydrogen or helium. Used extensively for research involving hypervelocity projectiles, the use of light gases as a propelling medium has produced projectile speeds up to several times greater than the highest speed attained by guns utilizing conventional propellants such as modern gunpowders.
In the prior art there exists various designs of light gas guns that can generally be categorized as being of one-stage, two-stage, or three-stage design. All three types of light gas gun designs are capable of firing projectiles at hypervelocity speeds. The object of this invention relates to the two-stage design. It should be mentioned that another type of hypervelocity gun appearing in the prior art is the shock wave gun, which in some embodiments takes the form of a special type of two-stage light gas gun.
In the two-stage light gas gun design, either hydrogen or helium gas is initially held within a so-called pump tube. Within the pump tube is a piston called the pump piston that is used to compress the light gas. Rigidly connected to one end of the pump tube is a so-called launch tube that holds a projectile to be launched. An explosive charge, such as gunpowder or a fuel/air mixture, lies on one side of the pump piston. On the other side of the piston is the light gas along with a diaphragm that initially prevents the light gas from flowing from the pump tube into the launch tube. The diaphragm, which is placed near the junction of the pump and launch tubes, is a type of one-use valve that is designed to burst open at a preset pressure. When the explosive charge is ignited it causes the piston to accelerate towards the diaphragm, an action that quickly compresses the hydrogen (or helium). When the piston has compressed the light gas to a predetermined pressure, the diaphragm bursts open. The high-pressure, hot hydrogen (or helium) pours through the burst diaphragm and into the launch tube, which in turn causes the projectile to be expelled from the launch tube's muzzle. The launch tube is typically several times smaller in diameter than the pump tube. The pump and launch tubes together form the overall length of a conventional two-stage light gas gun.
In the NACA technical note 4143 by Charters et al (1957) a two-stage light gas gun is described that contains a pump piston as well as a heavier secondary piston called a valve piston. After ignition of the powder charge, the pump piston and valve piston are driven in opposite directions along the length of the pump tube. The movement of the heavy valve piston allows the delayed release of hot propellant gases from the pump tube. The pump piston is designed to ‘bounce back’ after the diaphragm is ruptured, preventing it from ramming into the end of the pump tube, which could possibly damage the gun. In spite of its positive features, this design has several drawbacks. First, between firings the gun must be partially disassembled in order to return the pump and valve pistons to the firing position. Another drawback is the residue—such as a carbon buildup—that forms due to the repeated use of a solid propellant in the pump tube, which must periodically be cleaned out. Another disadvantage is that the pump tube must be lengthened in order to accommodate the rearward movement of the valve piston. A final drawback is that a danger exists that if too much propellant is used, or an insufficient quantity of light gas is present before firing, that the freely-moving pump piston will collide with the end of the pump tube, leading to damage of the piston, the pump tube, or both.
In U.S. Pat. No. 2,872,846 Crozier (1959) shows an alternative embodiment that is basically identical to the Charters (1957) design described above, except that Charters' valve piston, whose action allows leftover propellant to escape the pump tube, is absent. The simpler design, however, leads to its first drawback: there is no provision for the automatic and convenient venting of propellant gases once the gun has been fired. Other than that difference, Crozier's design has several distinct disadvantages in common with Charters' design. First, a danger exists that if too much propellant is used, or an insufficient quantity of light gas is present before firing, the pump piston will collide with the end of the pump tube, leading to damage of the piston, the pump tube, or both. Second, between firings the gun must be partially disassembled in order to return the pump piston to the firing position. Third, due to the repeated use of a solid propellant in the pump tube, residue forms that periodically must be cleaned out.
In contrast to the design of Charters et al (1957) summarized above in which the pump piston bounces back from the end of the pump tube, U.S. Pat. No. 2,882,796 to Clark et al (1959) describes a pump piston designed to purposely ram into the diaphragm-end of the pump tube. The pump piston is made of a material—such as nylon—that is readily deformable under high pressures. This design has the advantage that it eliminates the concern of damage to the pump tube by the pump piston, since the pump piston is specifically design to impact and then squeeze into the constriction of the pump tube that leads into the launch tube. However, there are distinct disadvantages of this design: 1) as in the Crozier (1959) design described previously, there is no mechanism provided to automatically vent the remaining propellant gases once the gun has been fired; 2) the pump tube must be opened up so that the tightly squeezed compression piston can be extricated, considerably slowing the process of preparing the gun for another firing; 3) after each firing, residue from the propellant can contaminate the interior of the pump tube; and 4) after each firing the old pump piston is severely distorted and must be discarded, while a new pump piston must be loaded into the pump tube. Discarding the pump piston after each shot increases costs as compared to a pump piston that can be reused repeatedly.
In U.S. Pat. No. 4,038,903 Wohlford (1977) describes a telescoped two-stage light gas gun. The telescoped gun was intended as an anti-aircraft weapon, its design permitting a higher rate of fire as compared with previous two-stage light gas gun designs. The gun is designed so that the pump piston and launch tube always move together as a single, ridged unit. One favorable feature of the gun is that the area of the pump piston that the propellant gas pushes against is greater than the area of the pump piston that compresses the light gas; unfortunately, the ratio of propellant area to compression area is not very high, being only fractionally higher than unity, i.e., much closer to a ratio of 1 than to a ratio approaching 2 or more. In spite of a few favorable features, the telescoped design suffers from a number of drawbacks: 1) in order for there to be a good seal between the outside of the gun barrel and the inside of the pump tube opening, not only must the inside of the gun barrel be machined to a high degree of precision (which is normally the case for most gun barrels), but also both the outside of the gun barrel and the inside of the pump tube opening must be machined very close to round as well. However, repeated firing of the weapon will heat its various parts. If the gun barrel is heated more or less than the pump tube, the expansion of the two parts will also vary, which could lead to either significant loss of gas at the pump tube/launch tube seal, or to increased friction at the same seal thereby slowing the motion of the pump piston; (2) this design allows propellant residue to form on both the inside of the pump tube and the outside of the launch tube, which can lead to increased wear of those parts, as well as the need for frequent cleanings of those same parts; (3) after a projectile is fired from the gun, the reloading of another projectile is overly complicated. First the rear of the pump tube must be opened, and then the rear of the launch tube must be opened as well. After the projectile (and possibly a diaphragm) is loaded, first the launch tube must be closed, followed by closure of the pump tube. Such a procedure takes an inordinate amount of time for a gun designed to be a weapon; (5) if too little light gas is introduced into the pump tube, then the pump tube piston might violently collide with the end of the pump tube housing, damaging or destroying the gun; and (6) in the telescoped gun design, the breach end of the launch tube is rigidly connected to the pump piston. That pump piston/launch tube connection is riddled with holes that allow the hot, compressed light gas to enter from the pump tube. Such a design is structurally much weaker than in other light gas gun designs, wherein there is a simple transition from the pump tube into the launch tube, and said transition of the two tubes is very strong because it is encased within a large block of metal.
In U.S. Pat. No. 4,658,699 Dahm (1987) describes a two-stage light gas gun referred to as a ‘wave gun’. The wave gun uses a light and flexible pump piston that—after the projectile has exited the launch tube—is forced through the pump tube/launch tube constriction, and then travels through and out the launch tube. Higher muzzle velocities of the projectile are claimed for this design, as compared to other two-stage light gas guns. The design, however, is beset by a variety of drawbacks: (1) expulsion of the light piston entirely from the gun means that propellant residue contaminates not only the pump tube, but the launch tube as well; (2) the mechanical integrity of the pump piston is questionable because it is designed to travel back and forth within the pump tube several times before finally being expelled from the gun. Such a ‘wave’ motion with the hot, high-pressure propellant gas on one side and the hot, high-pressure light gas on its other side would put enormous stresses on such a light and deformable piston, which could well lead to a blow-by of the propellant and/or light gases and subsequent contamination of the light gas with propellant, which in turn would degrade the interior ballistic performance of the projectile; (3) increased erosion of the launch tube interior. High velocity light gas guns have traditionally suffered from erosion of the launch tube after each firing of the gun. But the wave gun not only expels the projectile and associated light gas from the launch tube, but the pump piston and the propellant as well. The additional material ejected through the launch tube at high speeds would probably increase launch barrel erosion significantly as compared to more conventional designs; and (4) a final drawback of the wave gun design is that if all or part of the deformable pump piston does not completely leave the launch tube, its presence could impede a subsequent firing with potentially catastrophic damage to the gun.
In the article titled “World's Largest Light Gas Gun Nears Completion at Livermore” appearing in Aviation Week and Space Technology/Aug. 10, 1992/pp 57-59, a two-stage light gas gun designed by John Hunter uses a methane/air mixture as the propellant to accelerate a heavy steel piston down a long pump tube to compress the light gas. The pump tube is at a right angle to the launch tube. Shock absorbers negate the recoil transmitted through both the pump and launch tubes. The pump and launch tubes are connected in such a way that the launch tube can be swiveled to any angle from horizontal up to vertical. A positive feature of Hunter's design is that it uses a clean-burning and inexpensive propellant source. However, the design possesses a number of disadvantages: (1) the pump tube is excessively long compared to the launch tube length; indeed, the prototype that was constructed had a pump tube nearly twice as long as the launch tube. Such a long pump tube makes for an unwieldy design, and means a much more expensive gun; (2) a right angle between the pump and launch tubes leads to large torques on each tube that are eliminated with shock absorbers, which increases complexity and the total cost of the gun. Moreover, failure of a shock absorber could lead to severe damage of the gun, especially in the vicinity where the pump and launch tubes meet; (3) even though methane is typically very clean burning as compared to, say, gunpowder, if the combustion of methane is not complete, carbon deposits could still form in the pump tube; (4) after the gun is fired the freely-moving, heavy pump piston must be returned the length of the long pump tube before another firing can take place, slowing the time between firings; and (5) the swivel connection between the pump and launch tubes, which allows a projectile is to be fired at various angles, must be made of very strong materials and to very close tolerances so that no leakage of hot gases occurs, which all translates into a significant increase in the cost of the gun.
In NASA Contractor Report 4491 titled “Concept Definition Study for an Extremely Large Aerophysics Range Facility” by Hallock F. Swift, dated February 1993, a two-stage light gas gun is proposed that foregoes the use of a combustible propellant to propel the pump piston, using instead helium compressed to 15,000 pounds per square inch. The helium is held within high-pressure storage tanks until it is quickly released into the pump tube, at which time the highly compressed helium accelerates a large and heavy pump piston down the pump tube, compressing low-pressure helium on the opposite side of the pump piston, which in turn launches the projectile from the launch tube.
A prominent feature of the proposed light gas gun is that no propellant residue should form in the pump tube since the propelling gas—namely helium—is non-combustible. In spite of that advantageous characteristic, the design has a number of other features that are decidedly disadvantageous: first, the pump piston is partially deformed on each shot, and must be either discarded completely, or repaired for subsequent use, and either option translates into increased cost per shot from the gun; second, at the end of each firing the pump tube must be opened and a device inserted in order to retrieve the used pump piston, a procedure which considerably slows the process of readying the gun for another firing; third, helium used as the propelling gas of the pump piston is rather expensive; therefore, the design calls for reuse of the helium, which entails pumping it from the pump tube back into the original storage tanks; the reuse of the helium increases the complexity of the entire gun system, and greatly delays the possible time between firings; the author cites a ballpark figure of around an hour to recompress the helium; while higher-capacity pumps could certainly decrease the time needed to recompress the helium, the higher initial and ongoing costs associated with their use would also significantly increase the overall cost of the entire system.
As demonstrated above, there are many different designs of two-stage light gas guns known in the prior art. Each design possesses various strengths and weaknesses, some of which were outlined above; however, the designs known heretofore all suffer from a number of drawbacks:                (a) after the gun is fired, the pump piston cannot be quickly returned to its original start position for another firing of the gun;        (b) the length of time to reload the gun with a projectile is excessive;        (c) in the prior art a number of different types of gases have been used to propel a pump piston down the length of a pump tube, but under the right conditions any type of propelling gas is capable of leaving residues within the pump tube that build up over repeated firings of the gun;        (d) after the gun is fired, the spent propellant gas is expelled either through the use of some type of valve integrated into the pump tube, which adds cost and complexity to the gun design, or by exiting through the launch tube, which can foul the launch tube with propellant residue and/or increase interior erosion;        (e) the area of the pump piston the propelling gas pushes against versus the area of the pump piston that compresses the light gas is restricted in all previous designs known heretofore, and that restriction limits the utility of those designs; specifically, most designs in the prior art set the area of the pump piston that the propelling gas pushes against equal to the area of the pump piston compressing the light gas; but at least one design results in a ratio of propelling area to compression area slightly greater than 1; however, no known previous design allows a broad range of ratios.        (f) no design known heretofore is easily adapted to a variety of roles; a design that is well-suited for laboratory research is unwieldy when applied to a military role or space launch applications, and vice versa;        